ITU-T G.707 describes the advantages offered by SDH and multiplexing method, and specifies a set of SDH bit rates, the general principles and frame structure of the network node interface(NNI), the overall frame size of 9 rows by N*270 columns, the section overhead(SOH) together with its byte allocation, arrangements for international interconnection of synchronous transport modules(STMs), the formats for multiplexing and mapping elements into the STM-N at the NNI.
The North America equivalent of SDH is SONET. SONET is the U.S.(ANSI) standard for synchronous data transmission on optical media. People ensure standards so that digital networks can interconnnect internationally and that existing conventional transmission systems can take advantage of optical media through tributary attachments. SONET defines a base rate of 51.84 Mbps and a set of multiples of the base rate known as Optical Carrier Levels. The SONET is an octet-synchronous multiplex scheme that defines a family of standard rates and formats. Despite the name, it is not limited to optical links. Electrical specifications have been defined for single-mode fiber, multi-mode fiber, and CATV 75 ohm coaxial cable. The transmission rates are integral multiples of 51.840 Mbps, which may be used to carry T3/E3 bit-synchronous signals. It is also strong recommended to use G.703 E1/E3/E4/T1/E2/T4 interfaces as physical layer of IP-over-SDH/SONET. It is convenient user access way via LAN.
Both SDH and SONET provide standard for a number of line rates up to the maximum line rate of STM-192/OC-768. Actual line rates approaching 40 gigabits per second are possible.
When SDH/SONET ADM (Add and Drop Multiplex) is applied in a ring shown in FIG. 1, there exist the following defects:    (1) Long configuration cycle is needed for end-to-end trail connection of High Order Virtual Container and Low Order Virtual Container via DCC (data communication channel) (Regeneration section D1-D3 bytes, Multiplex section DCC D4-D 12 bytes);    (2) There are multiple equipment layers: Regeneration section, Multiplex section, High Order Virtual Container and Low Order Virtual Container. So developed system and network is complicated so much.    (3) It is expensive and inefficient for packet to map to HOVC (High Order Virtual Container), LOVC (Low Order Virtual Container) or their contiguous concatenation and virtual concatenation of SDH/SONET systems.    (4) Support of synchronous clock network is needed heavily, otherwise performance of system will be degraded or go down.    (5) Support of TMN (telecom management network) is also needed, otherwise it is no way for interworking for maintenance and operation with other vendors product.
When LAPS (Link Access Procedure—SDH/SONET) in X.86/Y.1323 or GFP (Generic Framing Procedure) is applied for Ethernet over SDH/SONET in a ring of SDH/SONET ADM (Add and Drop Multiplex) shown in FIG. 1 and FIG. 2, there exist the following defects:    (1) Bandwidth of Generic Framing Procedure and Ethernet/Gigabit Ethernet is mismatched. The use of virtual concatenation of HOVC and LOVC increases the complexity of equipment.    (2) It is difficult to implement dynamic bandwidth allocation of packet based.    (3) Only point-to-point connection is supported. It is no way to provide multicast and broadcast functions.
When RPR (Resilient Packet Ring, shown in FIG. 3) is applied in MAN (Metro Network Area) shown in FIG. 3, there exist the following defects:    (1) It is difficult to implement functions of Adding and dropping tributary for renting.    (2) Accessing to RPR node needs layer 3 forwarding of network processor or Layer 3 packet switching (e.g. crossbar, shared memory etc). Cost of node equipment is very high.    (3) Support of multiple services (Ethernet, Gigabit Ethernet, DVB etc) in a node of ring needs to be located to everything over IP. So the use of multiple protocol layers will lead to difficulty in performance.    (4) RPR does work by sending data packets in outer ring and corresponding control packet in inner ring, and data packets in inner ring and corresponding control packet in outer ring. But in case of single fiber cut (e.g. outer ring), corresponding control packet sending in an inner ring does not know that.